A method of making cooking utensils can have the plate, the sheet-metal disk and the container base centered and joined by electric resistance heating to produce a central spot weld, after which the intermetallic bond is produced by single or multiple pulse pressure and plastic deformation; the cap collar can be formed on the sheet-metal disk and its rim is placed around the container. Of course this process, on which the invention is based, and consequently the method according to the invention also, can be carried out at ambient temperature or with heating.
The impulse energy will be adjusted to the temperature of the container, at least in the base region, the cap and the plate. Usually these temperatures are chosen so as to be near but sufficiently below the melting-point of the plate material. Aluminum, in the form of industrially pure aluminum, is a very useful plate material. Copper is also a useful plate material. The cooking utensils can be pots, pans or the like, more particularly, cooking pots and pressure cookers. The plate improves the heat transfer from a hotplate or stove plate to the food. The heat transfer is adversely affected if the intermetallic bond is damaged. This applies particularly when a defect occurs in the collar region since experience shows that it can propagate from there when the utensil is in use.
The known method on which the invention is based (German Pat. No. DE-PS 22 58 795) does not result in a cooking utensil having a cap base which has a convex curvature towards the container. Instead, the process yields a substantially flat cap bottom. Use is made of a flat sheet-metal disk, the edge of which cannot and is not designed to be permanently deformed against the container when the collar is formed on the cap. The aim is only to shape or bevel the edge of the sheet-metal disk for closely fitting to the container. To this end, the disk diameter is less than the rated outer diameter of the pot. The impulse pressure shapes the flat cap bottom, forming the collar in the same process. It has been found in practice that this cannot ensure a faultless intermetallic bond between the cap, the cap bottom, the cap collar and the plate or between the plate and the container base. Frequently, the centering obtained before impulse pressure, is destroyed and the product has to be rejected.
In another known method (European patent application no. Ep 0 209 745) the procedure is as follows: the plate is secured by spot welding to the base of the preformed cap. The assembly comprising the cap, plate and container base or container is heated to a temperature which is near but below the melting-point of the plate material.
The initial thickness of the plate is at least 20% greater than the final thickness of the plate after the intermetallic bond has been formed.
In a first phase of pulse pressure, the pressure is applied progressively from the center to the edge of the assembly so as to obtain a special convexity, i.e. so that the total curvature of the convexity of the shaped plate and of the container is not less than 0.5% of the average diameter of the container base in its final shape. This impulse driving air from the center outwards, out of the assembly comprising the container base, plate and cap.
The known method, however, results in constraints which restrict the usefulness of the previously-described steps. This is because of the conditions, i.e. on the one hand the initial thickness of the plate must be at least 20% greater than the final thickness thereof after manufacture of the intermetallic bond, and on the other hand the convexity must be adjusted as previously described.
Also it is not clear from the reference that there should be a spot-welded connection at the center and that the connection should be resistant to impulse pressure.
If the spot-welded connection breaks during pulse pressure, it is impossible to prevent displacement during the pressure pulse. Consequently these can be no guarantee of an intermetallic bond meeting all requirements even at the rim of the plate and at the collar on the cap.
In reality, however, it is very important to have a faultless intermetallic bond overall, particularly in the region of the cap bottom (compare the periodical "Materialprufung" 33 (1991) 4, pages 89 to 91).
There has not been available an automated process for ensuring a bond in systems of the type described, unless the starting material is a relatively flat sheet-metal disk instead of a preformed enclosing cap.